DC at the input, scratchy guitar volume pot

[Silvio55]

What are your supply, plate, and cathode voltages for that stage?

Do you have a scope? Check for oscillation.

Mark, I was cheking just that, and they are a little low, i have 125V at the plates of V1, supply is 215V, and the cathode votages are 0.76V, the big voltage drop in the supply happens after the fist supply resistor (2 10K in series in the schem, I replaced them with 1 22k resistor), it goes from 428V to 278V through that resistor. Can the problem be related to low voltage at the plates?

[nickb]

Well, after reversing the bias cap my amp worked, but Iīm having this little problem now, my guitar volume pot is scratchy with this amp, so I looked for DC at the input of the amp and it was there, -0.8VDC in stand by and -0.3VDC with the amp on. I tried changing tubes but the problem is still there. What can be the problem? What should I check first?

I feel the need to point out that DC on the input of the amp will have no effect on scratchy pots. DC on the pots, however, is a different thing altogether.

[markr14850]

Sometimes when the supply voltage to the preamp tubes is set very low, they will start to draw grid current, and DC appears. But 215V is high enough that I wouldn't expect that to be the case here.

Your cathode voltage seems a bit low, which will have the bias set a bit hot. Is your cathode resistor the correct (measured) value?

If everything else about the amp is wonderful, maybe you could just try a .1uf film cap on the input to block the dc.

[R.G.]

Just to add to the confusion, 0.8uA = 800nA

Oh, sure - get the decimal point right just to make me look bad... It was the bane of my existence before calculators when all I had was a Log-Logf Duplex to get the decimal point in the right place. Now I can only blame doing math in my head.

@OP on further notes:
Weird. I sure thought that if you're getting measurable voltage, it was leaking from the tube grid, as there isn't supposed to be anything else connected for it to come from.

DC back into the guitar for sure will make the guitar pots sound scratchy. I've seen a number of complaints about this, and fixing the DC fixed the scratchiness. Just not this odd form of DC.

I can tell you a fix, but it only fixes the symptom, not the underlying cause. String a 2.2M resistor across the tip and ring at the input jack. Change the grid leak resistor to 2.2M, and run a film capacitor between input jack tip and the grid. The cap blocks the DC, no scratchy guitar pot.

This bothers me, because it does not fix the problem, and now the leak can build up and cause issues with the tube and its amplification. But not knowing what is causing this, it might be OK.

It's probably worth a few minutes with a soldering iron.

[Silvio55]

Sometimes when the supply voltage to the preamp tubes is set very low, they will start to draw grid current, and DC appears. But 215V is high enough that I wouldn't expect that to be the case here.

Your cathode voltage seems a bit low, which will have the bias set a bit hot. Is your cathode resistor the correct (measured) value?

If everything else about the amp is wonderful, maybe you could just try a .1uf film cap on the input to block the dc.

Yes, the bias is a bit hot in both channels, V1a and V1b use a 820R cathode resistor each, I measured them and they are ok

Oh, sure - get the decimal point right just to make me look bad... It was the bane of my existence before calculators when all I had was a Log-Logf Duplex to get the decimal point in the right place. Now I can only blame doing math in my head.

@OP on further notes:
Weird. I sure thought that if you're getting measurable voltage, it was leaking from the tube grid, as there isn't supposed to be anything else connected for it to come from.

DC back into the guitar for sure will make the guitar pots sound scratchy. I've seen a number of complaints about this, and fixing the DC fixed the scratchiness. Just not this odd form of DC.

I can tell you a fix, but it only fixes the symptom, not the underlying cause. String a 2.2M resistor across the tip and ring at the input jack. Change the grid leak resistor to 2.2M, and run a film capacitor between input jack tip and the grid. The cap blocks the DC, no scratchy guitar pot.

This bothers me, because it does not fix the problem, and now the leak can build up and cause issues with the tube and its amplification. But not knowing what is causing this, it might be OK.

It's probably worth a few minutes with a soldering iron.

I know that a cap at the input will cure the symptom, but as you said, it bothers me too, itīs like a band aid repair, but if I canīt find the cause, I guess it's the only thing I can do...

[drewl]

Why are you connection the input directly without an input grid resistor?
Even Marshall used a cap on some models.

I had this problem with a Weber Tweed kit someone built.
It had input resistors, but still had dc out to the guitar.

Nothing I tried worked, so I think I gave up and used an input cap.

[Silvio55]

Why are you connection the input directly without an input grid resistor?
Even Marshall used a cap on some models.

I had this problem with a Weber Tweed kit someone built.
It had input resistors, but still had dc out to the guitar.

Nothing I tried worked, so I think I gave up and used an input cap.

Because input gridstoppers can add noise, and thereīs another way for filtering RF, that is making the grounding point right at the input jack (itīs Paul Rubyīs method), as I said it worked wonderfully in my other amp, no noise and no RF. I tried adding a gridstopper too, but the problem persisted, tried every tube I had in hand, and always the same result, itīs frustrating! I have no idea where does this DC come from, maybe a plate leakeage, but with every tube? I donīt know, maybe bad luck with all my 12ax7's...

[Silvio55]

Why are you connection the input directly without an input grid resistor?
Even Marshall used a cap on some models.

I had this problem with a Weber Tweed kit someone built.
It had input resistors, but still had dc out to the guitar.

Nothing I tried worked, so I think I gave up and used an input cap.

Because input gridstoppers can add noise, and thereīs another way for filtering RF, that is making the grounding point right at the input jack (itīs Paul Rubyīs method), as I said it worked wonderfully in my other amp, no noise and no RF. I tried adding a gridstopper too, but the problem persisted, tried every tube I had in hand, and always the same result, itīs frustrating! I have no idea where does this DC come from, maybe a plate leakeage, but with every tube? I donīt know, maybe bad luck with all my 12ax7's...

[Enzo]

Sounds like plain old grid leak to me. Charge from the cathode can build up on the grid, creating a small negative voltage. A input cap is not a "band aid", it is just the solution.

This phenomenon is the basis for grid-leak bias, which you can look up. Many older amps used this. ANy of those input stages where the cathode is directly grounded, and the grid resistor is VERY high, like 5 meg.

[Silvio55]

Sounds like plain old grid leak to me. Charge from the cathode can build up on the grid, creating a small negative voltage. A input cap is not a "band aid", it is just the solution.

This phenomenon is the basis for grid-leak bias, which you can look up. Many older amps used this. ANy of those input stages where the cathode is directly grounded, and the grid resistor is VERY high, like 5 meg.

Well I installed an input cap now, .1 uf and problem is gone, but I wonder why every other amp I played that donīt have an input cap donīt show this problem, and why most tube amps I know donīt have an input cap, I can only remember the Orange Tiny Terror having one.